(1). Field of the Invention
The invention comprises an implantable prosthesis having at least two portions which are displaceable relative to each other, in particular a prosthesis for the upper end of a human upper arm comprising a shaft which can be inserted into the humerus passage thereof and a dome-shaped joint head which is to be associated with the joint socket adjacent to the upper arm and which can be connected to an end face of the shaft in an eccentric position relative to the center axis thereof by means of an intermediate disk which has a circular periphery and which comprises both in relation to the joint head and in relation to the shaft a respective insert pin or the like peg-shaped portion formed thereon which can be inserted into a bore. The invention also concerns the use of displaceable portions and an intermediate disk on a displacement plane.
(2). Description of the Related Art
A joint prosthesis of that kind is to be found in EP 0 549 480 A1. A central insert pin projects from the intermediate disk at each of the two sides thereof. Provided for the insert pin at the head side in the joint head is an eccentric bore and around same a ring of smaller bores for receiving a fixing pin which is parallel to the axis and which engages both into the bore and also into the intermediate disk. Displacement of the joint or humerus head is effected stepwise and cannot therefore be adapted to the factors involved in all situations. In addition the structure of this shoulder joint prosthesis does not make it possible to displace section axes or resection axes.
EP 0 712 617 A1 also describes a joint prosthesis, in the shaft of which the joint head is supported by means of a ball joint. This also does not permit fine tuning of the joint head.
In consideration of that state of the art, the inventor set himself the aim of so designing a prosthesis of the kind referred to in the opening part of this specification that, in regard to adaptation thereof to the respective anatomical aspect of the human bones which are to be connected therewith, fine adjustment is also possible without any problem.
That object is attained by the teaching of the independent claim; the appendant claims set forth advantageous developments. Furthermore, the scope of the invention embraces all combinations of at least two of the features disclosed in the description, the drawing and/or the claims.
In accordance with the invention the mutually displaceable portions are arranged steplessly displaceably around axes relative to each other at at least one displacement plane, wherein preferably one of the insert pins is to be arranged at a radial spacing relative to the axis of the intermediate disk.
By virtue of those measures, the shaft and the joint headxe2x80x94or two parts which are to be correspondingly displaced of other implant prosthesesxe2x80x94can be displaced at that end face or parallel thereto steplessly in relation to each other.
In accordance with another feature of the invention the intermediate diskxe2x80x94which can be inserted into an opening of the same shape as same in one of the adjacent portionsxe2x80x94is to be fitted with a central insert pin into a corresponding opening formed eccentrically in a connection plane of the joint head; the otherxe2x80x94eccentrically disposedxe2x80x94insert pin is associated with the end face of the shaft. The connection plane moreover determines one of two displacement planes of the prosthesis; the other lies in the end face of the shaft.
It has proven to be desirable for the diameter of the intermediate disk to approximately correspond to half the diameter of the joint head and for the height thereof to approximately correspond to the respective height of the insert pins thereof; for that purpose the height of the intermediate disk should be equal to approximately a third of its diameter with a preferred pin diameter whose length in relation to the diameter of its intermediate disk is in a ratio of between about 1:2.5 and 1:3.0. In addition the radial spacing of the eccentrically disposed insert pin from the axis of the intermediate disk should approximately correspond to half the pin diameter.
It will be clear that the connecting surface of the joint head according to the invention affords a shallow blind opening which is displaced out of the axis of the connecting surface, for insertion of the intermediate disk which is equipped with an insert pin on both sides thereof. The two insert pins of the intermediate disk are displaced radially relative to each other and make it possible for the joint head to be steplessly displaced with respect to the prosthesis shaft.
Preferably the insert pins are fitted as a press fit into their blind bore. For example they can each afford an annular press-fit region whose height at a tenth of the pin height is sufficient but may also be wider.
In a further embodiment the insert pin may be of a slightly tapering configuration; the conicity of the insert pin also guarantees the seating thereof in the cylindrical blind bore.
In accordance with a further feature of the invention the position of the intermediate disk is fixed by assembly thereof to the joint head or the shaft respectively.
In a specific embodiment of the invention the diameter of the insert pins can measure between 1 and 9 mm, preferably up to 7 mm. For that purpose then the intermediate or eccentric disk is of a diameter of between 10 and 35 mmxe2x80x94preferably between 12 and 30 mmxe2x80x94and a thickness of between 2 and 8 mm, preferably between 3 and 7 mm.
With dimensions of that kind, it has proven to be advantageous if the shallow opening for the intermediate disk in the dome-shaped joint head of the implantable prosthesis is displaced out of the center axis thereof by up to 10 mm, preferably up to 8 mm.
The association in accordance with the invention of two displaceable portions and the intermediate disk thereof at a displacement plane can also be used for implantable prostheses of another kind.